The present invention relates to the use of active resistors in the control of the loop gain of an active circuit.
It is known to provide a gain circuit using input transconductors in combination with an amplifier. A transconductor is a circuit which receives an input voltage and generates an output current. The magnitude of the output current of the transconductor is proportional to the input voltage received. The ratio by which the output current changes for a given ratio of input voltage change is known as the conversion gain, or transconductance (GM), of the transconductor. A differential transconductor receives a differential voltage impressed between two input terminals (ignoring the common-mode voltage) and generates a differential current on two current output terminals.
FIG. 1, labeled prior art, shows an example of a circuit which performs a gain or filter function. The circuit includes an input transconductor (GM1) which converts the voltage received at its input to a differential current at its outputs, an amplifier (A), which amplifies the signal provided by the input transconductor, feedback resistors (RF1 and RF2), feedback capacitors (CF1 and CF2) and a termination resistor (R), which is provided at the input of the summation amplifier. Such a circuit may be used as a gain or filter stage in more complex linear circuitry.
For stability, the magnitude of the loop gain of a circuit must be less than one before the phase shift around the loop exceeds 360.degree.. In the case of the FIG. 1 circuit, if the output impedance of amplifier A is much less than the value of the resistors RF1, RF2, then the loop gain is equal to the open loop gain of amplifier A. Thus, if amplifier A is stable, the loop gain the circuit including feedback resistors RF1, RF2, feedback capacitors CF1, CF2 and input transconductor GM is stable.